After reading up online a bit, it seems this error appears on PHP 5 servers because of class name conflicts. So I opened up nusoap.php file and figured that you could just use nusoap_client as a class name. Code piece from nusoap.php

The above is Drupal 6 version. First I wrote one for Drupal 5 and it just didn’t work for me, until some nice fellow from StackOverflow pointed out the difference between 5 and 6 when it comes to handling user hooks.